Aircraft: CITATION EXCEL (C560XL)
Phase of flight: Parked
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor

We preflighted in ZZZ1. We had flown that aircraft for a few days already. We noticed a puddle of oil on the ramp underneath the APU drain. There was oil steadily leaking from the drain. Upon further inspection we saw oil; coming from the APU titanium box. There was oil all over the lines coming from the box and on the compartment door. We called maintenance and Aircraft on Ground (AOGd) the aircraft; and also emailed pictures of the different oil smears and puddles. Maintenance came out to look at the aircraft and told us that the APU probably was over serviced and that he cleaned out the excess oil with alcohol. We asked how that was possible because the APU does not take a whole lot of oil; so whoever over serviced it must have poured a whole quart of oil all over the APU; seeing the amount of oil leakage. But the aircraft was released and we got a brief update to go to ZZZ2 and fly passengers to ZZZ3. Those flights were done without any issues; and the APU was used. Upon arrival in ZZZ3 we got a brief update to ferry to RYY. Again; the APU was used. Upon landing in RYY we shut down the engines and started up the APU. I was in the cabin when all of a sudden the cabin and cockpit filled with smoke. I immediately shut down the APU and evacuated the aircraft. I called maintenance and told them of this event. Maintenance told me to please start the engines without the APU to isolate the source; and then we could MEL the APU. I replied that we would do that but we were going to wait a while until all the smoke clears out of the aircraft. We did not want to breathe in the fumes. We then started the engines and ran them for a bit and indeed no smoke was observed and the APU was MELd. We were scheduled for this same aircraft the next afternoon. The crew that was assigned to the aircraft in the morning was still there. They told us that they were not comfortable flying it after a smoke event; because there was no inspection done on it to see if the APU was indeed the source of the smoke. They could not AOG the aircraft because the APU was 'legitimately' MELd already. We went out to preflight it; and right away we did find a puddle on the ramp again underneath the drain pipe. Upon further inspection we found the same oil leaks as yesterday coming from the APU titanium box. I then called the Assistant Chief Pilot (ACP) on duty and informed him that the aircraft had to be AOGd again for the same issues. I sent pictures to maintenance again and also the ACP.This is a gray area. We did not think the APU was over serviced because of the amount of oil that was leaking yesterday. However when maintenance looked at it and worked on it; we had no further reason not to take the aircraft. And granted; it flew fine and the APU worked fine for a couple of legs. I completely understand the sentiment of the crew that felt uncomfortable flying on the aircraft because it was just 'assumed' that the source came from the APU. The whole time that that crew was trying to deal with this issue; there was maintenance on the field working on another aircraft for an APU problem as well! It could have been easy to have that maintenance crew look at our aircraft also and locate the source of the smoke. Instead the company opted to do nothing and have us come out to take the aircraft that a previous crew did not fly for good reasons. We could not refuse this aircraft for the MEL alone; but there was an obvious oil leak again to AOG the aircraft once more.As I am typing this report I can see that maintenance now found the oil leak. The APU was not over serviced; but they found an internal leak in the APU. After any smoke event our aircraft should be inspected; there should be a procedure that covers this. We were already working more than 14 hours (duty) when his happened yesterday; maybe that clouded my judgment. Because I think I should have pursued an AOG and inspection for this aircraft and not just an MEL. That would have saved the other crewalso from the predicament they found themselves in this morning. We arrived at RYY and started the APU to facilitate us with the post flight. As I was doing the outside post flight the APU was suddenly shut down. So I went back to see what happened only to see a lot of smoke coming out of the cabin. My partner had shut down the APU and evacuated the airplane. We talked with Maintenance and they told us to startup the engines to 'Isolate' the bleed and see if it still smokes so we can MEL the APU. So we waited for about an hour for the smoke to subside and did the test. There was no more smoke coming from the outlets so we MEL'd the APU. That same morning we already wrote up the APU for excessive leakage in the [drain] hole and the drain pipe. (it literally had a puddle of oil underneath the APU drain pipe).This morning when arrived at the FBO we met the crew that was assigned to our airplane earlier and told us that they were uncomfortable to fly that airplane since no maintenance had inspected the airplane. So the company dumped the airplane back in our laps; only to find out that again there was a puddle of oil underneath the APU drain. So we had to Aircraft on Ground (AOG) the airplane again.First of all I don't like to be put in a situation where a previous crew didn't feel comfortable flying the airplane so we give it to the next crew. The company should take safety concerns at heart from any crew member; especially when smoke in cabin is involved. Second I cannot understand why there is no inspection done after a smoke in the cabin incident. Just to 'isolate' the bleed is not enough; that should be a quick fix to get the airplane on the ground. A thorough inspection for the root cause of the smoke; before clearing the airplane back to the line should be the proper and safe way for a smoke procedure.I don't blame Maintenance since their hands are tied by the inadequate procedures set forth in the manual. During preflight; a Citation crew noticed a puddle of oil beneath the APU drain. Maintenance diagnosed over-servicing as the problem and cleaned up the excess oil. The aircraft was re-released and the crew flew two subsequent flights using the APU during ground ops. After landing on a third flight; the crew started the APU and shut down the engines at the gate. The cabin began to fill with smoke and the aircraft was shut down and the passengers evacuated. Maintenance MELd the APU. A Subsequent crew refused the fly the aircraft. Maintenance then discovered an internal oil leak in the APU.